The present invention relates to a sensing or measuring apparatus which includes a flexible signal transmitter, e.g., an optical fiber, and a flexible sensing element. More particularly, the invention relates to such an apparatus and method which involve means to reduce the detrimental effect of the flexibility of the signal transmitter and sensing element.
Sensors, e.g., optical sensors, are quite useful in systems to measure the concentration of a component in a medium. For example, optical sensors can be effectively employed for measuring or monitoring a given component in blood. Typical components measured by sensors include gases, such as oxygen and carbon dioxide, hydrogen ions (pH), electrolytes, glucose and the like.
Such sensors include an indicator, e.g., a fluorescent dye, which interacts with the component to be sensed or measured. Typically, the indicator, often in combination with a component permeable matrix, is a sensing element or sensor means and is placed on or adjacent a surface of a signal transmitter e.g., an optical fiber. The interaction between the indicator and the component to be sensed or measured is monitored utilizing signals carried by the signal transmitter.
The size of such sensors, which in certain instances are sufficiently small to be employed in vivo in the treatment of a medical patient, often results in the signal transmitter and sensing element being flexible. As used herein, an item is "flexible" if the application of a force perpendicular to the longitudinal axis of the item results in a displacement of the longitudinal axis. For example, a single optical fiber made of glass is normally flexible, as is the sensing element.
Although the signal transmitters and the sensing elements are typically flexible, such flexibility can cause substantial difficulties in obtaining accurate and consistent measurements. For example, such flexibility can result in the signal to be transmitted by the signal transmitter becoming distorted or otherwise not representative of the signal being provided by the indicator in the sensor. In addition, this flexibility may cause unwanted relative movement and separation of the signal transmitter and the indicator, which also may have a detrimental effect on the consistency and integrity of the signal being transmitted by the signal transmitter. In view of the size constraints imposed on such sensors, it is often not possible to avoid using flexible signal transmitters and sensing elements. However, it would clearly be advantageous to provide such a sensor in which the adverse effect of such flexibility is reduced. For example, it would be advantageous to reduce the probability of relative movement between the signal transmitter and the sensing element.
A number of U.S. Patents and an European Patent Publication have been considered in preparing this application.
Costello U.S. Pat. No. 4,682,895 discloses a probe which includes a light sending optical fiber, a light receiving optical fiber, a sample chamber filled with colorimetric substance, a tip support coating which extends proximally of the faces of both optical fibers, a protective sheath which terminates proximally of both of these faces and a semipermeable membrane. Costello teaches the tip support coating, and not the protective sheath, as providing a rigid protection for the tip of the probe. Also, Costello's system which involves two fibers (derived from a doubled-up single optical fiber) may be too bulky for certain applications.
Gudmunder, et al European Patent Publication No. 0,047,094 discloses an optical measuring system including a dual channel probe. Light is passed through the probe by one optical fiber to a fluid-solid interface to be measured which causes the interface to fluoresce or phosphoresce, and that resulting light is passed back through the probe by another optical fiber and analyzed. A plastic sheath surrounds middle portions (neither distal end nor proximal end) of the optical fibers, and is bonded to a flexible cable to join the probe to the cable and form a hermetic seal to prevent the ingress of contaminants. The sheath does not extend beyond either end of the optical fibers and therefore does not protect or support such ends.
Smolinsky, et al U.S. Pat. No. 3,864,019 discloses an unclad optical fiber firmly attached to a transparent substrate by photopolymerization of a transparent filter, e.g., cyclohexyl methacrylate, located between the unclad fiber and the substrate. Schoch, et al U.S. Pat. No. 4,639,594 discloses a fiberoptic probe which includes two optical fibers, a light source and two photomultipliers. Lindgren U.S. Pat. No. 4,516,020 discloses a system in which an optical fiber is surrounded by light transmitting glass or plastic. The system is used to detect undesired, light-producing events. Peterson, et al U.S. Pat. No. 4,200,100 discloses a probe including two optical fiber and a flexible protective sheath which extends well beyond the distal ends of the fibers. Peterson, et al U.S. Pat. No. 4,476,870 discloses a two optical fiber probe which includes a dye packing and a hydrophobic gas permeable porous envelope to isolate the dye packing from contamination. None of these systems is concerned with reducing the harmful effects of the flexibility of a sensing element and a signal transmitter, in particular a single optical fiber.